1. Field of the Invention
The present invention relates to an over-sampling type A/D converter and, more particularly, to an A/D converter capable of performing conversion at a high accuracy.
2. Description of the Related Art
In a conventional A/D converter, it is considered that the number of quantized samples is increased to improve conversion accuracy. However, in order to perform this, a large number of code discriminators must be used, and an A/D converter having a large-scale arrangement must be used. For this reason, the following A/D converter is proposed. That is, in this A/D converter, upper bits are roughly determined by a discriminator, and lower bits are determined by this discriminator. In addition to the above A/D converter, an over-sampling type A/D converter represented by a .DELTA.-.SIGMA. modulator has received a great deal of attention as A/D converter in which an A/D conversion characteristic having higher accuracy than that of a conventional A/D converter can be obtained by using 1-bit (binary) A/D & D/A converters. This A/D converter is a high-accuracy A/D converter suitable for an integrated circuit. That is, in this converter, even when an analog circuit element does not have high accuracy, an analog-to-digital conversion characteristic having high accuracy of, e.g., about 16 bits can be obtained. The .DELTA.-.SIGMA. modulator is described in the literature, e.g., "Over-sampling type A/D & D/A conversion technique (first to sixth)", Akira Yukawa, Nikkei Electronics, No. 453-460, 1989.
In order to use the .DELTA.-.SIGMA. modulator as a high-accuracy A/D converter, there are two conventional problems.
First, when an integration order is increased to third or more order to improve an S/N ratio by using a 1-bit D/A converter, an output signal cannot follow an input signal to cause conversion to be unstable. It is known that this drawback can be solved by using a multi-bit D/A converter. However, the multi-bit D/A converter requires, e.g., a high-accuracy analog circuit element having 16 or more bits, therefore, the .DELTA.-.SIGMA. modulator loses an important advantage for an IC, i.e., the .DELTA.-.SIGMA. modulator can be constituted by elements having no high accuracy. When a high-accuracy analog circuit element is used, A/D converters other than a .DELTA.-.SIGMA. modulator can be sufficiently used. Nevertheless, when a .DELTA.-.SIGMA. modulation type A/D converter is used, the .DELTA.-.SIGMA. modulation type A/D converter loses its merit, because an A/D converter having 16-bit accuracy which can be obtained with good reproducibility without trimming an IC pattern in an IC is almost limited to a 1-bit (binary) converter. In converters other than a 1-bit converter, even when the 16-bit accuracy can be obtained, an area occupied by the converter is to be excessively large.
Second, in the .DELTA.-.SIGMA. modulator, since the output voltage amplitude of a latter-stage integrator is increased more rapidly than that of a former-stage integrator to degrade a whole dynamic range. For example, an integrator must be able to output a voltage having an amplitude twice the maximum input range without distortion in a .DELTA.-.SIGMA. modulator for performing first-order integration, and an integrator must be able to output a voltage having an amplitude four times the maximum input range without distortion in a .DELTA.-.SIGMA. modulator for performing second-order integration. Therefore, a dynamic range is determined at a portion where the maximum amplitude is generated. Although this problem can be solved by using a multi-bit D/A converter, the above problem on the accuracy of elements occurs. When an IC is formed by the elements, a high-accuracy A/D converter cannot be obtained.
Therefore, when the S/N ratio of a .DELTA.-.SIGMA. modulator is to be improved, extensive studies for developing a circuit arrangement using only a 1-bit D/A converter have been made. For example, an effort of this development is described in the literature "Reduction of Quantization noise of 1-bit Over-Sampling Type A/D Converter", Ken Yoshitome and Kuniharu Uchimura, (lecture papers A-126 in the 1988's spring national meeting of the Institute of Electronics, Information and Communication Engineers). In this literature, the following example is disclosed. That is, although 1-bit resolution D/A converters are used in conventional first- and second-order .DELTA.-.SIGMA. modulators, a multibit quantizer is used as an A/D converter, and quantization noise caused by a difference between the number of bits of the multi-bit quantizer and the number of bit of the 1-bit D/A converter is removed by digital processing. Therefore, according to this method, an S/N ratio can be advantageously improved compared with a conventional .DELTA.-.SIGMA. modulator having 1-bit A/D and D/A converters. However, since the 1-bit D/A converter is used in this method, the second problem is not solved yet, and therefore, the first problem is not solved.
As described above, in a conventional .DELTA.-.SIGMA. modulation type A/D converter, the output voltage amplitude of a latter-stage integrator is increased more rapidly than that of a former-stage integrator to degrade a whole dynamic range. For this reason, a multi-bit D/A converter may be used to solve this problem. However, the A/D converter with a high accuracy can not be attained by the multi-bit D/A converter.